The evacuation of hollow spaces is required in many technical applications, for instance in the case of electric tubes, liquified gas pipelines and so-called vacuum insulations. The gaseous atmosphere present in the hollow space to be evacuated is drawn off by means of a vacuum pump which, depending on the required value of the vacuum to be applied, operates in accordance with different principles, for instance, such as a liquid jet pump, reciprocating pump, centrifugal pump. The required pumping time not only depends on the efficiency and the volume of the evacuation space but it is also strongly influenced by the geometry of the evacuation space and increases disproportionately the lower the pressure stage of the vacuum to be obtained. It is customary to heat the subject hollow body during the evacuation to temperatures of, for instance, 300.degree. C. in order also to remove during the evacuation the molecular water layers or gas layers adhering to the inner walls of the evacuation space or to solids which were introduced into the evacuation space for instance, such as a heat-insulating material.
For the dependable maintaining of a high vacuum for a lengthy period of time, such as several years, it is furthermore known to introduce so-called getter materials into the evacuated hollow space. These getter materials are solids and have the property of absorbing gases which are subsequently liberated within the evacuation space or penetrate into the space from the outside. One known agent for the purpose is activated charcoal. It is furthermore known from Federal Republic of Germany Patent No. 34 36 754 to use metal hydrides having a base of Ti-V-Fe-Al-Cr-Mn as getter material for maintaining a vacuum within the vacuum jacket of thermal insulating containers.
The vacuum is produced in this case by pumping. The quantity of metal hydride introduced into the evacuation space amounts to 2-4 g/dm.sup.3 of vacuum space.
In order to obtain the outstanding heat-insulating properties of vacuum insulation it is necessary to assure a high vacuum on the order of at least 10.sup.-3 to 10.sup.-4 mbar. The walls of a suitable insulating jacket are, as a rule, made of metallic materials, in particular of alloy steel. In order to enable a mutual supporting of the inner and outer walls of the insulating jacket and to minimize the heat losses due to heat radiation, the hollow space is frequently filled with porous insulating material, for instance, kieselguhr or fibrous insulating material such, for instance, as glass fibers. Although the addition of such materials reduces the volume of the gases which are to be removed from the hollow space during the evacuation, the pumping times for obtaining an equally good vacuum are greatly increased as compared with the time required for a corresponding empty vacuum space due to the large number of minute hollow spaces, for instance, pores formed by the heat-insulating material. While, for instance, for an "empty" vacuum space a pumping time of 30 to 60 minutes was required for a vacuum of 10.sup.-3 mbar, the pumping time for the corresponding "filled" vacuum space amounted to about 12 hours. In this way, however, orders of magnitude are reached which constitute an obstacle to the manufacture of corresponding heat-insulating elements in large series, not to mention mass production.
One method of evacuating electrical vacuum discharge is known from Federal Republic of Germany Unexamined Application for Patent No. 15 39 126, in which the removal of the gaseous atmosphere from the evacuation space takes place without pumping. The housing of the device to be evacuated is for this purpose inserted into a hydrogen furnace and heated at 450.degree.-500.degree. C. while continuously flushing with hydrogen, so as to remove all foreign gases and adherent impurities, which decompose into gases under the action of the temperature.
Before starting the heating, an evacuated capsule is inserted into the housing, the capsule being substantially filled with titanium powder. After sufficient flushing with hydrogen, the flushing openings present in the housing are hermetically sealed and the housing is cooled. By a device which can be actuated from the outside, the capsule containing the titanium is thereupon punctured so that the hydrogen contained in the housing has access to the titanium powder. Due to its hydride-forming property, the titanium avidly absorbs the gaseous hydrogen so that a vacuum is produced inside the housing. Due to the use of a capsule for the hermetic enclosing of the hydride-forming titanium and due to the required puncturing mechanism as well as due to the need for a special furnace with a hydrogen atmosphere, this method is very cumbersome and even dangerous (danger of explosion) and therefore poorly suited for large-scale manufacture.
For the preparation of the capsule a similar procedure is described in Federal Republic of Germany Unexamined Application for Patent No. 15 39 126, which also uses a hydrogen furnace. Into the capsule which is provided with a series of openings for the passage of the gas there is introduced, on a sieve-like intermediate level, a quantity of powdered titanium hydride which fills about half the volume of the capsule. The capsule is then inserted into the hydrogen-flushed furnace and heated to more than 700.degree. C. so that the hydrogen bound in the titanium hydride is practically completely liberated. Together with the hydrogen of the furnace atmosphere, the liberated hydrogen causes a thorough flushing of the inside of the capsule and the displacement of all foreign gas components. A further increase in temperature to about 1000.degree. C. leads to the incipient melting of hard solder disks which are arranged in the immediate vicinity of the flushing openings so that all flushing openings are hermetically sealed after the cooling of the capsule.
The enclosed hydrogen atmosphere is avidly absorbed by the titanium powder so that a vacuum is produced. However, this vacuum does not have any immediate function with respect to the subsequent use of the capsule for the evacuation of electrical apparatus, but serves merely to preserve the absorption capacity of the titanium powder. The vacuum within the capsule is thus merely an incidental or so-called "auxiliary vacuum" and not a "useful vacuum" actually to be produced in a comparatively much more voluminous hollow body. Also, due to the high heating temperatures required, this method is considered unsuitable for most applications for the evacuation of large hollow spaces since the materials of the walls of the hollow space would frequently change their properties in impermissible manner at such temperatures.